6 CHAPTER VICLINICAL ANATOMY OF THE BRAINSTEM. SIGNS OF LESIONS. BULBAR AND PSEUDOBULBAR PALSY. THE MEDIAL LONGITUDINAL FASCICULUS
The brain stem is a small, narrow region connecting the spinal cord with the rest of the brain. It lies ventral to the cerebellum, which it connects by the cerebellar peduncles. Its functions are critical to survival. The brain stem is densely packed with many vital structures such as long ascending and descending pathways that carry sensory and motor information to and from higher brain regions. It contains the nuclei of cranial nerves III through XII and their intramedullary fibers. In short, it is a complicated but highly organized structure that controls motor and sensory activities, respiration, cardiovascular functions, and mechanisms related to sleep and consciousness.
The medulla (bulb) is the direct rostral extension of the spinal cord. It contains the nuclei of the lower cranial nerves (IX, X, XI, and XII) and the inferior olivary nucleus. The dorsal column pathways decussate in its central region to form the medial lemniscus, whereas the corticospinal tracts cross on the ventral side as they descend caudally. Together with the pons, the medulla also participates in vital autonomic functions such as digestion, respiration, and regulation of heart rate and blood pressure.
The pons (bridge) lies rostral to the medulla and appears as a bulge mounting from the ventral surface of the brain stem. The pons contains nuclei for cranial nerves V, VI, VII, and VIII as well as a large number of neurons that relay information about movement from the frontal cerebral hemispheres to the cerebellum (frontopontocerebellar pathway). Other clinically pertinent pathways in the pons are those for the control of saccadic eye movements (medial longitudinal fasciculus [MLF]) and the auditory connections.
The midbrain, the smallest and most rostral component of the brain stem, plays an important role in the control of eye movements and coordination of visual and auditory reflexes. It contains the nuclei for cranial nerves III and IV. Other important structures are the red nuclei and substantia nigra. The periaqueduct area has an important but poorly understood influence on consciousness and pain perception. Significant bilateral brain stem lesions produce altered mental status or coma. As a consequence of the unique anatomic arrangements in the brain stem, a unilateral lesion within this structure often causes “alternating syndromes”, in which ipsilateral dysfunction of one or more cranial nerves is accompanied by hemiplegia and/or hemisensory loss on the contralateral body.
Magnetic resonance imaging (MRI) is the examination of choice for suspected brain stem lesions. It provides a highly sensitive and noninvasive method of examine the posterior fossa.